Non-aqueous electrolyte secondary batteries can realize high voltage and high energy density, so they have been researched extensively. Positive electrodes of non-aqueous electrolyte secondary batteries include a transition metal oxide or a transition metal chalcogenide, such as LiMn2O4, LiCoO2, LiNiO2, V2O5, Cr2O5, MnO2, TiS2, or MoS2. Such material has a layered or tunnel-like crystal structure into and from which lithium ions are intercalated and deintercalated. The negative electrodes typically include a carbon material. Although a carbon material typically has a relatively small capacity, it is capable of reversibly absorbing and desorbing lithium, thereby providing a battery that is excellent in terms of cycle life and safety. Thus, lithium ion batteries including a negative electrode using a graphite type carbon material have been commercialized.
However, the theoretical capacity of a graphite material is 372 mAh/g, and the theoretical density thereof is 2.2 g/cm3, both of which are relatively small. It is therefore desired that a metal material capable of realizing a higher capacity than that of a graphite material will be used for a negative electrode. Among metal materials, Si particularly has a high capacity of 4199 mAh/g (theoretical density: 2.33 g/cm3), and hence an extensive research and development studies have been underway.
Although Si is capable of realizing a high capacity negative electrode, it has a significant problem to be solved with respect to the charge/discharge cycle characteristics of the resultant battery. The problem is that during charge and discharge reactions, the absorption and desorption of lithium involves repeated expansion and contraction of Si, thereby increasing the contact resistance among particles in the negative electrode and degrading the current collecting network. Such degraded current collecting network becomes a major factor that causes shortening of charge/discharge cycle life.
In order to solve the above-mentioned problems, there has been proposed as a negative electrode material an alloy forming material (hereinafter referred to as “alloying material”) that is capable of reversibly absorbing and desorbing lithium and includes a solid phase A and a solid phase B. The solid phase A and the solid phase B have a different composition, and at least part of the solid phase A is coated with the solid phase B. The solid phase A contains silicon, tin, and/or zinc, while the solid phase B contains a Group 2A element, a transition element, a Group 2B element, a Group 3B element, and/or a Group 4B element. Also, according to this proposal, the solid phase A is preferably amorphous or of low crystallinity. See, for example, U.S. Pat. No. 6,090,505 and Japanese Laid-Open Patent Publication No. 2004-103340.
Also, in terms of improving cycle life, Japanese Laid-Open Patent Publication No. 2004-335272 proposes a negative electrode active material comprising an A phase composed mainly of Si and a B phase comprising a compound of a transition metal and Si, where at least one of the A phase and the B phase is amorphous or low crystalline.
The related art techniques mentioned above are capable of significantly suppressing the cracking of the alloying material upon expansion and contraction thereof. Therefore, these techniques are effective to a certain extent in suppressing the degradation of the current collecting network, which is a major factor causing the degradation of cycle characteristics. However, a detailed examination of these related art techniques has revealed that the techniques may not produce sufficient suppression of cycle characteristic deterioration when batteries are rapidly charged and discharged at a relatively large current.
Reducing the particle size of a negative electrode active material has also been examined. For example, a Si powder with a mean particle size of 1 to 100 μm (Japanese Laid-Open Patent Publication No. 2003-109590), 0.1 to 2.5 μm (Japanese Laid-Open Patent Publication No. 2004-185810), 1 nm to 200 nm (Japanese Laid-Open Patent Publication No. 2004-214055), or 0.01 to 50 μm (Japanese Laid-Open Patent Publication No. 2000-36323) has been proposed. The use of a negative electrode active material in the form of a fine powder allows alloying of lithium and Si to proceed evenly upon charge, thereby suppressing localization of the reaction. It is therefore possible to reduce volume expansion due to alloying upon charge and volume contraction upon discharge, so that the negative electrode is unlikely to get distorted. As a result, it is considered that charge/discharge cycling can be repeated in a stable manner.
However, a conventional negative electrode is produced using a negative electrode mixture. For example, the negative electrode of a coin-shaped battery is a pellet that is obtained by molding a negative electrode mixture into a disc-shaped pellet under pressure. The negative electrode mixture contains a negative electrode active material that causes an electrochemical reaction, a conductive agent that supplements the electron conductivity inside the negative electrode, and a binder that makes these materials stick together. If the mean particle size of the active material is small, the density of the negative electrode obtained by molding the negative electrode mixture becomes small. Thus, the energy density per unit volume becomes low and the battery capacity also becomes small.
Also, if the mean particle size of the active material is small, the irreversible capacity of the battery increases, so that the battery capacity is further lowered. Further, the small particle size of the active material increases the reactivity of the active material with moisture and other components in the electrolyte, thereby promoting gas evolution. Accordingly, this is disadvantageous to the cycle characteristics and storage characteristics.
On the other hand, in order to obtain a negative electrode with a higher density or suppress gas evolution, if the mean particle size of the active material is increased, the distribution of the active material becomes uneven inside the negative electrode. Hence, intercalation and deintercalation of lithium upon charge and discharge become uneven inside the electrode, which is disadvantageous to the cycle life of the battery.
That is, the proposals of U.S. Pat. No. 6,090,505 and Japanese Laid-Open Patent Publication No. 2004-103340 and No. 2004-335272 have a problem with respect to a rapid charge/discharge at a large current. The proposals of Japanese Laid-Open Patent Publication No. 2003-109590, No. 2004-185810, No. 2004-214055, and No. 2000-36323 have a problem with regard to the balance between capacity and cycle characteristics. Particularly when the negative electrode is produced by molding a negative electrode mixture into a pellet, it is difficult to obtain a non-aqueous electrolyte secondary battery with a high capacity and excellent cycle characteristics.